The present invention relates to a method of making organic electroluminescent devices and, more particularly, to using radiation-induced thermal transfer of organic materials to produce display elements.
In color or full-color organic electroluminescent (EL) displays having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color producing organic EL media is required to produce the RGB pixels. The basic organic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium may consist of one or more layers of organic thin films, where one of the layers or regions within a layer is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the light-emitting layer of the organic EL medium. Other organic layers that may be present in the organic EL medium commonly facilitate electronic transportation, such as a hole-transporting layer (for hole conduction) or an electron-transporting layer (for electron conduction). In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the light-emitting layer of the organic EL medium or the entire organic EL medium. There is also a desire to increase efficiency and stability to as high as possible.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. It has been difficult to achieve high resolution of pixel sizes using shadow masking. Moreover, there are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of nonfunctioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side it is extremely difficult to manufacture larger shadow masks with the required precision (hole position of xc2x15 micrometers) for accurately forming EL devices.
A method for patterning high resolution organic EL displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a donor substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the donor substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the donor substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Such a method is disclosed by Littman in U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553). However, it has been found that the properties of the organic display materials transferred by a laser can be inferior to those deposited by conventional vapor deposition methods. Such properties include uniformity, efficiency, and stability. Thus, in order to take full advantage of the patterning advantages offered by radiation-induced thermal transfer, a need remains to improve the properties of the transferred material.
It is an object of the present invention to provide a method for patterning the organic EL medium without the limitations imposed by the conventional photolithographic or shadow mask methods.
It is another object of the present invention to provide an improved method for patterning high resolution, full color, organic EL displays.
It is another object of the invention to provide a method for patterning high resolution color EL displays without the limitation of mechanical precision alignment and allowing for dynamic alignment and simple pattern changes.
It is also an object of this invention to increase the efficiency and lifetime of the organic EL displays.
These objects are achieved by a method for making an organic electroluminescent display device having an array of pixels disposed on a display substrate, comprising the steps of:
a) providing an array of first electrodes on a display substrate;
b) providing a donor element comprising a donor support, a radiation-absorbing layer over the donor support, and at least one organic layer over the radiation-absorbing layer, wherein said organic layer comprises the material or materials to be transferred to the display substrate;
c) positioning the donor element in transfer relationship to the display substrate patterned with an array of first electrodes;
d) heating the display substrate or donor element or both within a specified temperature range prior to transfer of the organic layer;
e) focusing and scanning a laser beam of sufficient power and desired spot size on the radiation-absorbing layer of the donor element to effect the transfer of selected portions of the organic layer from the donor element to designated areas corresponding to pixels on the display substrate in electrical connection with the first electrodes; and
f) providing a second electrode over the transferred organic portions on the display substrate.
The present invention provides an advantage in that the heating step prior to radiation-induced thermal transfer yields an organic EL display with higher efficiency and higher stability. This invention is particularly suitable for making full color, organic EL displays with high quality, good efficiency, and stability. By printing with a scanning laser beam, fine precision patterning of colored pixels can be achieved, enabling the production of high resolution displays.